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A roller coaster of a campaign 18 months in the making arrives tomorrow at a huge moment for Hillary Clinton and Donald Trump: a 90-minute debate, with much of the nation expected to tune in amid great uncertainty about what they'll see.

Virtually tied in recent national polls, both Clinton and Trump enter the debate as the two most deeply unpopular presidential candidates in modern history. Both hope to discredit the other, and both hope to emerge from the debate having burnished the public's view that they are better qualified to be commander in chief.

A roiling disagreement over the role of the debate moderator flared up Sunday, with Democrats arguing that a more activist "fact-checker" role is needed to rein in Trump's well-established pattern of factual misstatements.

But Janet Brown, the executive director of the Commission on Presidential Debates, seemed to side with the Republican nominee, saying in a television interview that "it's not a good idea to get the moderator into essentially serving as the Encyclopaedia Britannica." She added, however, that ultimately it will be up to tomorrow's moderator, Lester Holt of NBC News, to do the job as he sees fit.

Underscoring the unique nature of the combatants, Clinton's debate preparations included a focus on Trump's personality as well as the substance of what will be discussed onstage at Hofstra University in Hempstead, New York, according to several Democrats with knowledge of her campaign's approach.

A stand-in for Republican presidential candidate Donald Trump is seen in a television camera monitor as preparations continue for the big debate. Photo / AP

Clinton's team convened a meeting last month at which longtime aide Philippe Reines, the stand-in for Trump in her mock sessions, deeply studied Trump's personality to be able to parry with her as Trump might.

The meeting was one of several during which Clinton aides conferred for hours with outsiders who had been asked to offer advice about Trump's temperament, according to people familiar with the gathering. The objective was to understand how a man who has spent most of his life in the business world and prides himself on being a dealmaker might behave in a debate setting.

The stakes tomorrow could hardly be higher for both candidates. A new Washington Post-poll released Sunday shows likely voters split nationally 46 percent for Clinton and 44 percent for Trump, with Libertarian Party nominee Gary Johnson at 5 percent and Green Party nominee Jill Stein at 1 percent.

With barely six weeks remaining until Election Day, Clinton's camp - after a prolonged focus on trashing Trump - sees the debate as a chance for her to present what she actually hopes to accomplish as president and to ease voters' deep concerns about her likability and trustworthiness.

For Trump, his first one-on-one presidential debate offers an opportunity to demonstrate a command of the issues and to persuade voters clamoring for change that he is a credible alternative, his advisers say.

One of the biggest unknowns remains which Donald Trump will show up. While Clinton has a lengthy record of meticulous preparation and formidable performances, Trump has been more unpredictable. Sometimes, he is the freewheeling showman prone to controversial utterances; other times, with help from his campaign team's repackaging, he is a more sober and scripted candidate.

The first of three scheduled debates between Clinton and Trump is likely to have a full agenda. It comes amid heightened fears of terrorism, unrest over police shootings of African-American men and a slew of long-standing issues that sharply divide the major-party candidates, including immigration, trade, tax policy and foreign affairs.

Supporters of Clinton and Trump, including their running mates and campaign managers, fanned out across the Sunday television shows to put their spin on the tasks ahead and seek some psychological advantage.

Trump's campaign manager, Kellyanne Conway, seemingly acknowledged on CNN's "State of the Union" that her candidate was trying "to get into the head of Hillary Clinton" when he suggested Saturday on Twitter on that he had invited Gennifer Flowers, who has claimed to have conducted a long-running affair with Bill Clinton, to attend the debate.

Trump's tweet followed news that rival billionaire Mark Cuban, who supports Clinton, would be sitting in the front row.

Trump's running mate, Gov. Mike Pence of Indiana, later said categorically on CBS's "Face the Nation" that Flowers would not be there.

In their TV appearances, Clinton partisans said she has multiple goals tomorrow. Those include reminding voters of her long record of championing the interests of children and families and touting her agenda for helping the middle class - but also holding Trump accountable for assertions that independent fact-checkers have labeled false.

"She has a challenge because Donald Trump inveterately says things that aren't true," Clinton's campaign chairman, John Podesta, said on NBC's "Meet the Press. "She's got to be able to make that positive case but also not let Donald Trump get away with what he's likely to do, which is to make stuff up."

Sen. Tim Kaine of Virginia, Clinton's running mate, said he expects the format to explore the truthfulness of both candidates' claims.

"There's a real opportunity to hear somebody say something and then get into whether is that actually true or not," Kaine said on "Face the Nation."

Trump's team continued to press its case Sunday that fact-checking shouldn't be the responsibility of the moderator, however.

"I really don't appreciate campaigns thinking it is the job of the media to go and be these virtual fact-checkers and that these debate moderators should somehow do their bidding," Conway said on ABC's "This Week with George Stephanopoulos."

She also disputed the notion that Trump makes more frequent misstatements, saying Clinton's "casual relationship with the truth is well-known to Americans."

Former House speaker Newt Gingrich, a Trump supporter, said he believes the moderators should remain a "modest" presence in the debates.

"They're not running for president," Gingrich said on "Fox and Friends Sunday." "It's pretty stupid to think we're going to have this third candidate called the moderator, and that they're going to double-team Donald Trump."

The stage is set for the presidential debate between Democratic presidential candidate Hillary Clinton and Republican presidential candidate Donald Trump. Photo / AP

Brown, the head of the independent debate commission, did not issue a verdict on the controversy during an appearance on CNN's "Reliable Sources" but said that in the past, the role of the moderator has been to keep things moving and allow the candidates to call one another out for misstatements.

Clinton's camp also continued efforts of recent days to argue that the press and public shouldn't hold her to a higher standard than Trump because of Clinton's longer record in public service and more-detailed policy proposals as a candidate.

"I'm very concerned that Donald Trump will be graded on a curve," Clinton's campaign manger, Robby Mook, said on CNN's "State of the Union." "Just because he doesn't fly off the handle in the middle of this debate does not mean that he is prepared to be president of the United States. ... He needs to roll out specific plans about how's going to make life better for Americans."

Aides to Trump, whose preparations by all accounts have been less meticulous than those of Clinton, are hopeful that the debate will help close what polls have shown to be a credibility gap with Clinton, a former secretary of state, senator from New York and first lady.

As part of an effort to appear more disciplined in recent weeks, Trump has put an emphasis on new policy proposals, which were sparse during the primary season, and on reining in his freewheeling style at campaign rallies. It remains to be seen whether those efforts will be maintained throughout tomorrow's 90 minutes on stage.

"A victory for Donald Trump tomorrow night is answering the questions and showing America that he's ready to be president and commander in chief on Day One," Conway said on ABC's "This Week."

Trump surrogates also sought to raise expectations for Clinton's performance, talking at length Sunday about her public service while repeatedly stating that Trump has never participated in a one-on-one debate.

"The expectations on Hillary are very, very high," said Reince Priebus, chairman of the Republican National Committee, on "Fox News Sunday." "She's been doing this for 30 years. I think people expect her to know every detail. ... He's never run before, let alone been in a presidential debate."

Trump's biggest challenge might be staying on message, as the episode over Flowers's possible appearance at the debate demonstrated. Heading into the debate, Trump's tweet on the subject not only risked distracting from the candidate's message but could further alienate women voters, with whom Trump has struggled.

On Sunday, Pence said that the real estate developer was just joking.

"Gennifer Flowers will not be attending the debate tomorrow night," Pence said on "Fox News Sunday."

Conway, speaking on CNN, said that Trump has no plans to bring up Bill Clinton's marital indiscretions during the debate, saying viewers deserve and expect these candidates to be talking about the issues."

But, she added: "I'm not going to reveal what we have been doing in our debate conversations. But the fact is that he has every right to be defend himself."

Clinton aides, meanwhile, argued that the episode was a telling one about Trump.

"You saw his reaction, which is to do his favorite sport, which is to dive in the sewer and go for a swim," Podesta said on NBC. "He's kind of predictable: When you poke him a little bit, and he comes back and attacks whoever is doing it."